Optical communications systems using a light source and photodetector or signal regenerator optically coupled to each other by means of an optical fiber have reached a stage of significant commercial importance and technical sophistication. Data rates in excess of 10 Gbit/sec and transmission distances in excess of 1000 kilometers are routinely achieved. To reach these levels of importance and sophistication, many types of components have been developed and used.
To cover a long transmission distance, initial optical communications system detected optical signals and created electrical signals and then generated new optical signals using the electrical signals in a repeater. Newer optical communications systems use optical amplifiers to regenerate the signal. In such systems, the optical signals are optically amplified rather than being converted first into an electrical signal. The optical amplifiers are typically rare earth, such as erbium, doped optical fibers. See, for example, IEEE Photonics Technology Letters, pp. 727-729, Aug. 1991, for a description of an exemplary erbium doped fiber preamplifier. The fiber amplifier or preamplifier should amplify as little as possible of the amplified spontaneous emission(ASE); this can be accomplished with a filter that passes only the signal wavelength.
The preamplifier in the article mentioned also had a tunable optical filter which the authors stated could have any of several forms including a fiber Fabry-Perot filter. An exemplary tunable Fabry-Perot filter using optical fibers was described by Miller at the European Conference on Optical Communication, Sep. 16-20, 1990. Use of fibers permitted the size of the filter to be reduced as compared to the size of filters using bulk components; optical fibers can be spliced directly to each other. Tuning was obtained by temperature variations. Numerous uses for the filter were mentioned.
Later it was realized that two fiber Fabry-Perot filters connected in series could be advantageously used in some applications. See, for example, the article by Miller in Electronics Letters, 28, page 216, Jan. 30,1992 for a description of a two stage filter used in a wavelength division multiplexed system. See also, Applied Physics Letters, 59, pp. 2369-2371, Nov. 4, 1991, for a description of a tunable fiber ring laser using a tandem Fabry-Perot filter. The series tandem Fabry-Perot filter suffers from several drawbacks including, but not limited to: 1) Both filters must be tuned to the same wavelength; this requires complicated electronics. 2) The optical coupling between the two filters adds an additional optical loss to the system.